Objectives: To develop a plaque assay for activated T lymphocytes, to characterize which type of T lymphocyte is measured by it, and to study the development of specific T-cells upon immunization. Approach: A plaque assay for antigen activated lymphocytes has been developed utilizing the fact that resting lymphocytes are non- permissive for lytic RNA viruses such as VSV while activated lymphocytes produce virus. Using mitogens for B- and T-cells as well as means for eliminating B- or T-cells, the identity of the virus plaque forming cell can be established.